Human cases of avian flu worry WHO

Jan 16, 2004 (CIDRAP News) – The World Health Organization (WHO) today confirmed a fourth fatal human case of avian influenza in Vietnam and voiced concern that conditions there have a potential for triggering an influenza pandemic.

Test results today confirmed the presence of influenza A(H5N1) in a Vietnamese patient who died, the WHO said in a statement. Earlier this week, the same strain was confirmed in three other Vietnamese patients who died. The virus is the same one blamed for current outbreaks of highly pathogenic avian influenza in poultry in Vietnam, South Korea, and Japan.

Besides the four confirmed cases, another 14 people in Vietnam are reported to have had severe respiratory illness possibly caused by the avian virus. The official Vietnam News Agency reported Jan 14 that 18 people had the virus, and 13 had died.

The WHO said, "The simultaneous occurrence of large and highly fatal outbreaks of H5N1 in birds is considered unprecedented. WHO is concerned that these events may indicate that H5N1 is becoming established in this part of the world. Because comprehensive surveillance to detect all cases in bird species is difficult, the true geographical occurrence of the avian epidemic may not be fully appreciated at present.

"The outbreak in Vietnam is of particular concern as it suggests the presence of many conditions that are known to have favoured the start of influenza pandemics in the past. Foremost among these is the co-circulation of human influenza viruses with an avian strain that is prone to mutate and has high pathogenicity."

The statement went on to note that H5N1 viruses have caused severe human disease and deaths twice in the recent past. In Hong Kong in 1997, the virus infected 18 people and killed 6, prompting the preventive slaughter of all poultry in the region. And in February 2003, two people in Hong Kong were infected and one died.

The H5N1 strain has a tendency to pick up genes from influenza viruses infecting other species, including humans, according to the WHO. Widespread outbreaks in birds increase the chances of human exposure to the virus. If people already carrying an influenza virus become infected with H5N1, the two viruses could trade genes, potentially giving rise to a new viral subtype with human flu genes that could enable it to spread easily from person to person, the WHO said.

"Such an event would mark the start of an influenza pandemic," the WHO said in a Jan 15 fact sheet on avian influenza.

Reports indicate this hasn't happened so far. WHO officials have said they believe the human cases in Vietnam all resulted from exposure to sick poultry, not from person-to-person spread. Further, partial sequencing of viruses from the human patients in Vietnam this week showed that all the genes were of avian origin, "indicating that the virus had not yet acquired genes from the human influenza virus," the WHO said.

Past influenza pandemics are thought to have arisen in settings where people lived in close proximity to poultry and pigs, according to the WHO. Because pigs are susceptible to both avian and mammalian influenza viruses, they can provide a "mixing vessel" for the viruses, leading to new subtypes. Earlier this week, the WHO said there were reports that the avian virus had infected pigs and ducks in Vietnam.

Further, the recent episodes of avian influenza in humans suggest that humans themselves could serve as the "mixing vessel" for new viral strains, the agency said. After the 18 human cases of H5N1 infection occurred in Hong Kong in 1997, epidemiologic and genetic studies made it clear that the virus had jumped directly from birds to humans, the WHO said. A few healthcare workers acquired the virus from patients, but they didn't get severely ill.

The WHO said it and its partners are stepping up activities to reduce human deaths and disease due to H5N1, reduce the chance of a new influenza pandemic, and conduct urgently needed research. Laboratories in WHO's global flu network are doing molecular studies of viruses isolated from infected birds and humans in the affected countries. Surveillance for human respiratory disease in Asia has been increased, and the sacrifice of infected or potentially exposed poultry is a standard control measure, officials said.

In related news, more than 19,000 chickens on a farm in Taiwan were slaughtered Jan 15 because of an infection with H5N2 influenza, a less pathogenic strain than H5N1, according to an online report in the Taipei Times. Birds at 21 other farms within 1 kilometer of the affected farm were inspected and found to be healthy, the report said.